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Experimental study on the circulating-cavity flow and an innovative central baffle design in a steam generator
Nuclear Engineering and Design 360 (2020) 110495
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Experimental study on the circulating-cavity flow and an innovative central baffle design in a steam generator
T
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Wang Yua,b, Lu Dao-ganga,b, , Wang Congc, Cao Qionga,b, Li Xiang-bina,b, Zhou Shi-lianga,b a
North China Electric Power University, Beijing 102206, China Beijing Key Laboratory of Passive Nuclear Energy Safety and Technology, Beijing 102206, China c Research Institute of Nuclear Power Operation, Hubei, Wuhan 430223, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Circulating-cavity flow Steam-water two phase flow Steam generator Visualization experiment
Steam generator (SG) is the key equipment in pressurized water reactors (PWR), which transfers heat from primary circuit to secondary circuit and has the feed water vaporized into the steam. It is related to the safe, reliable and economical operation of the nuclear power plant. Many researches have been done on SG, including numerical simulation and experimental research. Since it is involved with complicated steam-water (two phase) flow in high temperature and high pressure, it is not easy to measure the key parameters such as pressure, temperature and void fraction, especially to carry out the visual observation. So the detailed working information of SG such as void fraction and flow pattern is still unknown, while this information is very important for the improvement of SG performance. In order to obtain the working information of SG and study the dynamic flow process in secondary side, a visualization scaled-down mock-up experimental bench was set up. Although its operation parameters (temperature and pressure) are much lower than the actual one in PWR, the internal dynamic flow process of SG in this facility is kept similar to the actual one by the scaling analysis and design. Appling the high speed camera, particle image velocimetry (PIV) and self-made optical fiber probes, two phase flow behavior including flow pattern, velocity and void fraction were collected and analyzed in the experiment. From the experiment, a circulation phenomenon is discovered; which is defined as circulating-cavity flow (CCF). At U-bend area, CCF flows from hot side to cold side, which may result in flow-induced vibration (FIV) and jeopardize the U-tubes. In order to limit CCF, a baffle installed in the middle of U-bend area is proposed to suppress CCF. The experimental results show that this baffle can effectively suppress the CCF. This paper may contribute to the design and safety of SG.
1. Introduction In most pressurized water reactors (PWR), the steam generator (SG) is installed vertically, has inverted U-tubes and can do natural circulation (Guangdong Nuclear Power Training Center, 2007), such as the SG in Guangdong Daya Bay Nuclear Power Station (GNPS) shown in Fig. 1 (Liu et al., 2019). SG is the key equipment for heat exchange at primary and secondary coolant circuits in nuclear power plants (NPP). As depicted in Fig. 1, in primary circuits, the coolant with high temperature enters from coolant inlet to inverted U-tubes, and then flows out from coolant outlet. In secondary circuits, the feed water with relatively low temperature enters from feed water inlet to feed water ring and flows through the downcomer (which is between the tube bundle sleeve and the lower shell) to tube sheet, and then feed water flows along the U-tubes. The feed water is heated by the surface of U-tubes,
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which makes most of feed water vaporized to steam. At last, the steam is separated by steam separator and steam dryer from steam-water mixture. In the process of heat transfer, there is complex phase transition at secondary side. In most researches on SG, people most concerned about the heat transfer process between primary circuit and secondary circuit, and the phenomenon caused by steam-water two phase flowing such as flow induced vibration (FIV). As we all know, thermal stress and vibration wear are main reasons causing the damage of SG U-tubes, according to the operation data statistics in NPP (Zang and Shen, 2003), a quarter of unplanned trip was caused by the breakage of SG U-tubes. Thus, in order to acquire the distribution and flow behavior of steam-water two phase flow in secondary side of SG, people mainly used numerical simulation and experimental research to analyze the thermo-hydraulic characteristics of SG. In numerical simulation, most simulations are based on porous
Corresponding author. E-mail address: [email protected] (D.-g. Lu).
https://doi.org/10.1016/j.nucengdes.2019.110495 Received 4 September 2019; Received in revised form 11 December 2019; Accepted 16 December 2019 0029-5493/ © 2019 Elsevier B.V. All rights reserved.
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Generation 2+. The research pointed that the gap between AVB and Utubes, AVB twist and unreasonable insertion depth were critical to wear resistance and would aggravate the FIV. Thus it can be seen that the effective tube support is essential for guaranteeing the service lifetime of U-tubes. There are many people researching the local fluid behavior in the process of heat transfer. Parul et al. (2018) did visualization experiments with high-speed photograph technique to characterize the forced convective nucleate boiling flows. Yun et al. (2010) focused on local two-phase flow parameters through a series of subcooling boiling flow tests in steam-water. Li et al. (2013) studied bubbles sliding behaviors at a visual experiment. There are other excellent researches such as references (Bouyahiaoui et al., 2018; Shen et al., 2018; Lee et al., 2018; Kanizawa and Ribatski, 2016; Takeshima et al., 2002), they most focus on the local region and the flow characteristics inside the tubes, these researches can help us know about the flow patterns on different velocities, the change of void fraction under different mass qualities and so on. But the comprehensive flow behavior in SG especially flow pattern at U-bend area cannot be learned. Furthermore, in actual operation of SG, the main working medium at secondary side is steam-water two phase flow. The research developed in air-water and Freon has quite difference from actual phenomenon. In experiment, direct observation of experimental phenomenon is beneficial to research, but limited by the high temperature and high pressure working condition in SG, the visual research is hard to carry out, so the flow behavior of steam-water two phase flow in SG is still unknown and there is not accurate and comprehensive description. There are few visualization experimental researches on the whole of the SG. Delgado et al. (2018) did a visualization research on a model of helical coil SG using particle image velocimetry (PIV) (Atkins, 2016). But this type of SG is proposed in small modular reactor (SMR), not applied in PWR. In view of the previous researches, it is better to use steam-water working medium and build a visualization experimental bench to observe the detailed flow behavior. Thus, in this study, referring to the prototype SG of GNPS, a scaled-down mock-up experimental bench was constructed to study the dynamic flow process of SG. In order to achieve visualization research, eight glass windows were installed on the bench. And electrical heating rods were utilized to replace primary circuits in SG, which can make water vaporized to steam. The high speed camera, PIV and self-made optical fiber probes were applied to observe and collect data (including flow pattern, velocity and void fraction). The experiment also can provide a benchmark for numerical simulation.
Fig. 1. Structure diagram of GNPS.
media approach which was first proposed by Patankar and Spalding (1974). A lot of simulation procedures such as ATHOS3, MARS, CUPID, and CUPID-SG have been developed and applied (Keeton et al., 1982; Jeong et al., 1999; Jeong et al., 2010; Park et al., 2014). And there are some studies (Cong et al., 2013, 2014) depending on commercial computational fluid dynamics (CFD) code such as ANSYS FLUENT. Although the numerical simulation was excellent, it is not all codes were validated. And in the research of numerical simulation, it is inevitable to use some assumptions and empirical correlations derived from experimental data, which causes that the study results are unreliable. As Ishii and Hibiki (2010) pointed, the unreliable physical models and empirical correlations (for inter-phase friction, heat transfers, and so on) restricted the applicability of the code. For example, in two-fluid model, the physical models and empirical correlations largely depend on the flow regimes. The different assumptions of flow regimes can influence the results. As the references (Hetsroni, 1982; Ishii and Chawla, 1979) showed that the different empirical correlations were for various flow regimes assumptions. Therefore, for widely application, further research on reliable physical models and empirical correlations are needed. Besides, the internal structure of SG is very complex, modeling and meshing are relatively difficult, which also causes it hard to practical application. In experimental research on two phase flow, the tests were usually carried out in nitrogen-water, air-water, Freon and steam-water (Taylor and Pettigrew, 2001; Cargnelutti et al., 2010; Ulbrich and Mewes, 1994; Hsu et al., 1998; Vinod et al., 2014). For example, Chu et al. (2011) conducted the experiment in air-water two phase flow at room temperature and atmospheric pressure condition. Takai et al. (2000) performed the experiment using Freon-123. They both studied the FIV characteristics of U-tubes in SG. There are some other researches focusing on the flow characteristics when tube support was installed. For example, Quan et al. (2019) did experimental study on tube dynamic characteristics and FIV response of tubes with tube support plates (TSP) or anti-vibration bars (AVB). Lalonde et al. (2010) researched the vibration behavior of U-tubes with AVB through experiment, one of the results showed that the vibration of clearance between U-tubes and AVB may lead to ineffective support. And Cui et al. (2016) made analysis about the influence of AVB on integrity of U-tubes based on SG of
2. Experiment 2.1. Experimental bench and test section (1) Experimental bench The experimental bench consists of three major parts: I. Test section (SG model), II. Water supply system, III. Data acquisition system. The water supply system is made up of deionized water tank, feed pump, valve, flowmeter and preheater (shown in Fig. 2). After deionized, the water is saved into water tank. The feed pump and control valve are used to adjust the rate of flow. (2) Test section The test section is scaled down from prototype SG of GNPS based on the scaling analysis which refers to traditional Hierarchical Two-Tiered Scaling (H2TS) method presented by Ishii and Kataoka (1984). During the scaling analysis for primary side, in order to ensure that the model and prototype have the same heat transfer mechanism, the size and structure of U-tubes and tube support plates are same to those of prototype. The friction numbers can be the same by adjusting the damping coefficient. And in secondary side, the time ratio is 1. There are some scaling criteria numbers can meet requirements, but another scaling 2
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Fig. 2. Schematic diagram of experimental bench. Table 1 The main design parameters of SG in GNPS and experimental bench.
Fig. 3. Structure diagram and entity graph of test section. 1-tube sheet; 2, 3, 4, 5, 6, 7-U-tubes containing six electrical heating rods; 8-two groups of anti-vibration bars; 9, 10, 11, 12-four tube support plates.
Parameters
Values of prototype SG
Values of model SG
Thermal load per set Total heat transfer area Pressure of outlet steam The flow of steam Temperature of outlet steam Temperature of feed water The number of U-tubes The material of U-tubes The external diameter of U-tubes The wall thickness of U-tubes The tube pitch The type of tubes support plate
969 MW·t 5429 m2 6.89 MPa 1938 t/h 283.6 °C 226 °C 4474 Inconel 690 alloy 19.05 mm 1.09 mm 27.43 mm Quatre-foil plum blossom
380 kW·t 15.552 m2 0.101325 MPa 529.2 kg/h 100 °C 42 °C 27 304 stainless steel
shape
internal dynamic flow process of SG. We adjusted the experimental parameters and the thermal power was set to 380 kW. This thermal power is too low, so the CHF and Post-CHF phenomena don’t occur. And we just mainly focus on and discover the circulating path; this thermal power doesn’t limit our research. The test section is an about 5.3 m × 2.0 m × 0.1 m sliced box structure depicted in Fig. 3 and the structure of tube support plate is displayed in Fig. 4. The material of Utubes is 304 stainless steel of which the heat-conducting property is
criteria numbers such as phase change number, heat source number and subcooling number that are hard to meet requirements. In order to well revealing the experimental phenomenon, the thermal power should be 6200 kW, which is difficult to achieve in our laboratory conditions. Hence, considering that this experiment places emphasis on the mechanistic study, and the purpose of the experiment is to reveal the
Fig. 4. Structure diagram of tube support plate. 3
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Fig. 5. The PIV system and measuring positions in Velocity measurements.
Fig. 6. The shooting photo by PIV and PIV Image data processing.
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Fig. 7. Image of calibrating length. Fig. 9. Schematic diagram of bubble image processing.
similar to that of Inconel 690 alloy. The feed water is deionized water, so the water chemical corrosion is not considered. The detailed experimental parameters are listed in Table 1. The working pressure of secondary side is one bar atmosphere, thus, the saturation temperature of steam is 100 °C, and the feed water is heated to 42 °C by preheater, which ensures the same subcooling temperature of secondary side. The electrical heating rods are installed inside U-tubes and utilized to replace the primary side of SG. In actual operation, from coolant inlet to coolant outlet, the capacity of heat transfer decreases along the U-tubes. Thus, the heating rods were divided into six sections with linearly decreasing heat flux. For observing the flow field, eight glass windows were installed before and after symmetrically. In the experimental bench, all water pipes wrapped with insulation layer but glass windows and stainless steel shell of model SG, so the main heat loss of bench are from the above two parts losing heat. Considering the heat conduction of glasses and shell, heat radiation and convective heat transfer between water and glasses, the total heat loss is calculated to about 5 kW, which was added to test working condition.
(2) PIV data processing In order to get reliable velocity field, each measuring position is shot three times, and the best photograph is selected as a basis for data processing. Fig. 6(a) displays a instantaneous flow field captured by high speed camera in the preliminary test. The tracer particles can be clearly seen. All photos are processed by software Insight 3G and Tecplot to obtain the final analyzable image and data. Fig. 6(b) shows flow field (area inside the red box in Fig. 6(a)) between two U-tubes after data processing. Considering some undesirable data points caused by photo partial distortion, a section of 30 mm × 10 mm between tubes is selected for data processing. The original velocity data from PIV system is just transverse velocity and axial velocity, so resultant velocity has to be 2 2 + vaxi . calculated by formula of vsum = vtrans (3) High speed camera image data processing The experiment applies Fastec TS3 type, 1280 × 1024 pixels high speed camera which can shoot 500 photos in a second. Before experiment, it needs to calibrate length with a ruler (shown in Fig. 7). After experiment, it needs to observe and analyze the motion path of flow field. As Fig. 8 displays, a random bubble moves in a short time △t*(△t*=t2-t1). The two photos at t1 and t2 are processed by using image processing software Image J to measure bubble motion distance, including transverse and axial distance. The time △t* and distances have been get, since then the motion angle, motion displacement and bubble motion velocity can be obtained, thus, from Fig. 9, the general flow direction can be known.
2.2. Measurements and data processing methods 2.2.1. Velocity field measurement (1) PIV system In experiment, PIV system is used to measure the velocity of flow field especially observing bubbles flow behavior with high speed camera. The PIV system mainly consists of measuring devices and tracer fluorescent particles. As shown in Fig. 5(a) and Fig. 5(b), the laser transmitter and high speed camera are put at the same side of the windows and the angle between them is 90°. The angle between incident laser and window is 30°, so is the angle between shooting surface and flow field. When calculating the velocity, the measured transverse velocity should multiply cosine 30°(u*=ucos30°). Some strong reflective areas are covered by a black curtain to reduce the influence of environmental light. The velocity measuring positions (namely 1–10 points marked in Fig. 5(c)) are selected below U-bend area as where the boiling phenomenon becomes obvious; and the high speed camera mainly captures the a-d areas as where the two phase flow phenomenon is most intense.
2.2.2. Void fraction measurement (1) Optical fiber probe Fig. 10 shows the void fraction measuring points in test section. These 12 points are in the third view window (counting from bottom up). The optical fiber probes insert the glass and are fixed to measuring points, which consist of five parts (exhibited in Fig. 11): S1-sensitive head of probe; S2-support tube of optical fiber and sensitive head; S3sealing gland; S4-signal transmission wire; S5-optical connector which
Fig. 8. Schematic diagram of bubble motion. 5
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Fig. 10. Void fraction measuring points.
Fig. 11. Schematic diagram of L-type optical fiber probe.
Fig. 12. Schematic diagram of calibration bench. 1-water pump, 2,5-regulating valve, 3-liquid flowmeter, 4-gas pump, 6-gas flowmeter, 7-pressure relief valve, 8,12-drainage valve, 9-acrylic plates, 10-stainless steel capillary array, 11three-layer stainless steel filter, 13–15-quick-closing valve, 16-drain pipe, 17water tank, 18,19-differental pressure meter.
Fig. 13. Schematic diagram of optical fiber probe.
bypass pipe is opened. The height of water column and air column between two quick-closing valves are measured to calculate the average void fraction. Each probe was calibrated three times. Finally, the average calibration error of all twelve probes is 5.707%, and this error is within the acceptable range, so experiments could be conducted.
connects optical fibers to photoelectron component. In order to make sensitive head of probe fully contact with bubbles in secondary side, part S2 is made into L-type. The part S1 is towards to bottom as to enhance the contact with bubbles.
(b) Void fraction data processing
(2) Optical fiber probe measuring data processing (a) Calibration of optical fiber probe
As Fig. 13 shows, the light is transmitted to the sensitive head of probe. Because refractive indices of gas and liquid are very different, the intensity of returned light varies greatly, which make it easy to distinguish gas and liquid. In optical fiber electric signal processing, when output signal is larger than the threshold value and smaller than the minimum noise voltage, it indicates that this signal is caused by bubbles.
Before using self-made optical fiber probe, it is necessary to calibrate the probe performance. Fig. 12 is the schematic diagram of calibration bench. In measuring area of the calibration bench, the optical fiber probe is installed between the two quick-closing valves. Water and air are utilized to replace steam-water two phase fluid. Keeping water and gas flow rate unchanged and flow pattern in a stable condition, the optical fiber probe is used to measure the average void fraction in a period of time. At the same time, quick-closing valves are closed and the 6
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Table 2 Experimental working condition parameters. Time interval Δt (μs)
Exposure time (μs)
Total flow rate (kg/s)
Initial temperature (°C)
Environmental pressure (Pa)
100 Working condition 1 2 3 4
450 Power (kW) 155 155 255 255
0.147 Flow rate at cold side (kg/s) 0.0735 0.0294 0.0735 0.0294
42 Flow rate at hot side(kg/s) 0.0735 0.1176 0.0735 0.1176
1.01325 × 105 Feed water ratio 1:1 1:4 1:1 1:4
2.3. Experimental test procedure
3.2. Flow phenomenon shot by high speed camera
Prior to the experiment, the tracer particles are injected into the deionized water tank at the ratio of 1:50 and are mixed up. Referring to Fig. 2, the valve and feed pump are firstly opened, according to the reading number of flowmeter, the valve is adjusted to a suitable angle to make the flux 0.147 kg/s. Then preheater and the drain valve of SG are opened, until the temperature of feed water reaches 42 °C, the drain valve is closed. Next the three-way valve is opened, according to reading number of rotor flowmeter, feed water is distributed into cold and hot sides. At the time of water level reaching the required value, feed pump is closed and electric heating rod bundles are started. When boiling phenomenon appearing, the feed pump is opened again; after the flow becomes steady, it is time to collect the flow phenomenon by PIV system and measure void fraction by optical fiber probe. The test was performed at the conditions listed in Table 2. The heat loss of 5 kW was added into all working condition. When collecting void fraction data, the steady state duration is 120 s.
In four shooting areas a-d, every flow field has its general flow tendency. Considering their flow tendency, when operation power is 155 kW and the feed water ratio is 1:1, three groups of bubbles were selected randomly in different times and were recorded their initial and final positions in the same amount of time. All photos are processed according to image processing method mentioned in part 2.2.1, one of these bubbles motion track is shown in Fig. 16. The selected area is wide so that the deflection angles and velocities can reflect the flow information. The bubbles in every group are 5, 10 and 15; due to the big shooting area a, here the selected bubbles are10, 15 and 20. The results show that the parameters in three groups have little difference and the variations are in normal range (shown at Table 3). The deflection angles and transverse velocities of three groups both can reflect flow movement and the following analysis is mainly referring to the second groups. Fig. 16(d) shows that bubble1 is at straight tubes area and is going to flow into U-bend area. At this moment, the fluid has flowed a long distance and has been heated for a while. Small bubbles come together and form larger slug flow. As there is fluid density difference between two sides in transverse direction and bubble buoyancy drags fluid to flow upward, bubble1 not flows along tubes vertically but towards top-right. Average deflection angle of ten bubbles in area d is 60.13°(shown in Table 3), which reflects that two phase flow in d area flows towards top-right. Then, Fig. 16(b) and (c) displays that fluid flows into U-bend area and scours U-tubes transversely, which makes heat transfer suddenly intensify and the number of bubbles increase. The bubbles motion become more actively, and then they join together to form stir flow. It is hard to tell apart a single bubble by this time. Meanwhile, when fluid flows upward, it receives resistance from the Utubes. This resistance is greater than the frictional resistance produced by flowing along the bend tubes. Besides, in the area below U-bend, the driving force resulted by density difference makes two phase fluid flow continuously from cold side to hot side. This driving force and the resistance from U-tubes bring about fluid flow along the bend tubes. When fluid flows into centre position of U-bend, fluid is cut by tubes. Stir flow is cut broken into blocklike flow that can be seen in Fig. 16(a). The transverse velocity at U-bend centre reaches maximum. At the root, middle and centre of U-bend area, average deflection angles of bubbles in the second group are respectively 36.92°, 13.55° and 13.08°, which complementally illustrates that fluid flows along the tubes. On the whole, the two phase fluid generally flows along the bend tubes after going into the U-bend area, and the flow direction is from hot side to cold side.
3. Experimental results and discussion 3.1. Flow characteristic at secondary side of SG As we all know, steam-water two phase flow is the primary phenomenon at secondary side in SG. The velocities of steam-water flow at ten measuring points are displayed in Fig. 14, it can be found that fluid does not completely flow upward along straight tubes, but accompanied with transverse flow. And the velocities of points 1–5 is greater, which is because there is more intense heat transfer near U-bend area that directly affects flowing actively. The velocity streamline shown in Fig. 14 illustrates that fluid generally flows from cold side to hot side, and the transverse velocity increases along the direction of cold side to hot side as shown in Fig. 15. The main reason is that intense heat transfer at hot side produces many bubbles, which results in the density of hot side smaller. So in the large space of the secondary side, density difference brings about transverse flow. At cold side, what is worth noting that the axial velocity of the second row is greater, especially when the feed water ratio (the ratio of feed water injected into the cold side to that into hot side) is 1:1. This is mainly because when two phase fluid flows into U-bend area, fluid scours tubes transversely not just flows upward, which leads to more heat transfer in this area and then steam volume increases quickly. At the moment, the flow resistance of two phase fluid decreases, and massive bubbles carry water to move along U-bend to cold side. This motion generates a downward disturbance which has a big influence on the axial velocity of the first row, so the axial velocity at cold side is smaller. This feature is more obvious at the power of 255 kW. Axial velocities of point 1 and point 2 become negative values, which mean that fluid flows downward. When the feed water ratio is 1:4, heat transfer per unit fluid decreases at hot side but increases at cold side, so the density difference between two sides decreases, which results in transverse velocity tends to stable and the downward disturbance weakens relatively so that the axial velocities of point 1 and point 2 are positive values. As a whole, below the U-bend area, two phase fluid flows from cold side to hot side. And at cold side, there is a downward disturbance from U-bend area.
3.3. Void fraction field distribution The void fraction data of three rows are exhibited in Fig. 17. At the power of 155 kW, the void fraction is 0 at cold side, which may be because there is not obvious boiling at cold side so that departure from nucleate boiling (DNB) points only appears on several heating tube walls, and these generated bubbles are not fully developed, thus, in steady measuring stage, the measured data is 0. In comparison, heat transfer becomes more intense at the power of 255 kW; as a consequence, void fraction at cold side has a bit of promotion. And when 7
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Fig. 14. Velocity of ten measuring points. 8
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Fig. 15. Component velocity in transverse and axial directions.
the feed water ratio is 1:4, at cold side, void fractions of row1 and row2 are more than 10% and void fraction of row3 is at around 5%. This is as feed water is less at cold side; the heat transfer per unit fluid increases, which leads to more intense boiling. At hot side, heat transfer is relative much; numerous bubbles generate and combine together, so steam volume gradually increases. Due to the fact that heat transfer per unit fluid at hot side is more under the feed water ratio of 1:1, in this condition, void fraction changes faster. Table 4 displays average void fraction data. At the same power, void fraction distribution unevenness decreases with increase of uneven feed water ratio, that is to say, more feed water are injected into hot sides that can make the transverse void fraction field become uniform. As a whole, under the influence of fluid density difference in secondary side and more heat transfer at hot side, the void fraction at hot side is more than that at cold side.
in this large space brings about orderly flowing. When two phase fluid flows into U-bend area, resistance from bend tubes and driving force from density difference compels fluid to flow along bend tubes, here plenty of steam goes into steam separator, other two phase fluid flows toward cold side. According to the fluid motion parameters of shooting areas in Fig. 18(a), the fluid from the cold side would flows downward, and meanwhile they meet the below fluid which brings upward momentum, this momentum impedes the flow and compels them to change flow direction, so the fluids from two directions join together and then flow to hot side continually in the process of heat transfer. In the flow process, there is an obvious cyclic flow phenomenon at the U-bend area (shown in Fig. 18(b)), which is called circulating-cavity flow (CCF). The CCF may result in FIV and do harm to heat transfer tubes.
3.4. Analysis of circulating-cavity flow phenomenon
In order to suppress the CCF at U-bend area discovered in experiment, two types of central baffle are designed and set in the middle of U-bend. The central baffles are 5 mm thick steel plates, detailed parameters and installation positions are shown in Fig. 19. Through above analysis, when the feed water injected into two sides is the same, two phase fluid flows more intensely, hence FIV is more severe. Thus, based on this feed water ratio condition, the suppressing CCF test was performed. The flow phenomenon before and after installing central baffles are displayed in Fig. 20 and Fig. 22, where the red circle marks the position of one bubble at t1 and t2; the transverse velocity contrast results are shown in Figs. 21 and 23. As Fig. 20 displays, at the power of 155 kW, after installing baffle, most fluid is blocked to the hot side. Because the gaps between tubes are not sealed in baffle 1, there are some bubbles flowing to cold side through these gaps. In comparison, Fig. 20(c) shows that the bubbles
4. Suppression of the circulating-cavity flow
Through the above analysis, two phase fluid flow tendency in secondary side is summarized and estimated as Fig. 18 depicts. The feed water experiences a complex flow process in the secondary side of SG. First, feed water flows into downcomer of test section and then comes into contact with U-tubes. At this moment, fluid is still water. Then, fluid flows along the U-tubes and is heated a period of time. The degree of subcooling gradually decreases, when it reaches 0, namely the temperature of fluid reaches saturation temperature; a few bubbles appear on the wall of some individual tubes. Along with heat transfer increasing, more bubbles appear and combine together. As flow resistance of gas is small, bubbles could drag water flow upward. Meanwhile, at hot side, more bubbles and high temperature make two phase fluid density becomes smaller than that at cold side. Density difference 9
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Fig. 16. Bubble motions at four shooting area of U-bend area. Table 3 Average velocity of three groups of bubbles. Position/Bubble numbers
Displacement (cm)
Velocity (cm/s)
Transverse velocity (cm/s)
Deflection angle (°)
Time internal (ms)
a (10/15/20) b (5/10/15) c (5/10/15) d (5/10/15)
6.59/7.06/6.72 2.96/2.97/2.64 3.54/3.27/3.67 3.45/3.11/3.22
82.40/88.74/83.97 49.25/49.41/43.92 58.96/54.41/61.25 37.60/31.06/34.19
69.41/76.26/70.64 48.09/47.59/41.73 31.99/32.62/32.64 13.16/15.17/15.28
14.34/13.08/14.59 9.98/13.55/13.37 37.54/36.92/38.29 69.54/60.13/60.28
80 60 60 100
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Fig. 17. Void fraction of three rows.
transverse velocity decreases from 110.84 cm/s to 75.59 cm/s and 30.56 cm/s. In this test section, there are two groups of AVBs, and the baffle 1 is similar to AVB in structure. They both have gaps in radial direction after installed among the U-tubes. On the contrary, the baffle 2 is designed that it can fill up these gaps. Through the above velocity data and bubbles flow phenomenon, it can be found that the AVBs can weaken the CCF, but the suppression effect is not very good because there is fluid flowing through gaps. However, the baffle 2 obviously can suppress the transverse flowing well. Expect for supporting the U-tubes, this kind of baffle can both weaken vibration and reduce the flow channels. The baffle 2 can be considered as an optimization design in the future designs work.
Table 4 Average void fraction data. Power (kW)
155 255
Feed water ratio
1:1 1:4 1:1 1:4
Transverse average void fraction (%) Row 1
Row 2
Row3
Overall average value
10.972 9.698 23.813 15.713
9.605 7.139 19.231 10.880
4.415 3.819 12.449 7.004
8.331 6.885 18.497 11.199
flowing to cold side is less in SG with baffle 2. In Fig. 21, the transverse velocity decreases from 70.92 cm/s to 28.99 cm/s and 19.33 cm/s respectively after installing baffle 1 and baffle 2, which illustrates that the suppressing function of baffle works well and the restraining transverse flow effect of the baffle 2 is more significant. At the power of 255 kW, the two phase fluid flows more intensely, thus, more bubbles flow to cold side driven by the CCF (shown in Fig. 22), although like this, the baffle still has a certain suppressing effect. As Fig. 23 displayed, the
5. Conclusions The main conclusions of this present study are summarized as follows.
Fig. 18. Schematic diagram of the CCF flow behavior. 11
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Fig. 19. Two types of central baffle.
Fig. 20. Image of two phase flow behavior before and after installing central baffles (P = 155 kW).
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Fig. 21. Transverse velocity contrast on three conditions (P = 155 kW).
Fig. 23. Transverse velocity contrast on three conditions (P = 255 kW).
(1) A visualization scaled-down mock-up experimental bench is set up, which can reveal the internal dynamic flow process of SG and discover the circulation path of fluid in SG. (2) The CCF is discovered in experiment, which is a circulating flow phenomenon at U-bend area. Its detailed flow behavior (including flow pattern, velocity and void fraction) is obtained by PIV and selfmade optical fiber probes quantificationally. (3) Since the CCF may cause FIV and have bad impact on U-tubes. A baffle plate installed in the middle of U-bend area is newly proposed to suppress the CCF. The experimental results show that the baffle
can effectively suppress the CCF. Thus, the designer can consider designing a suitable baffle in SG to suppress vibration of U-tubes. CRediT authorship contribution statement Yu Wang: Data curation, Formal analysis. Dao-gang Lu: Conceptualization, Supervision, Visualization. Cong Wang: Investigation, Data curation, Formal analysis. Qiong Cao: Project administration. Xiang-bin Li: Formal analysis. Shi-liang Zhou: Resources.
Fig. 22. Image of two phase flow behavior before and after installing central baffles (P = 255 kW). 13
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Declaration of Competing Interest
code, MARS 1.3.1. Ann. Nucl. Energy 26, 1611–1642. Jeong, J.J., et al., 2010. The Cupid Code Development and Assessment Strategy. Korea Atomic Energy Society. Kanizawa, F.T., Ribatski, G., 2016. Two phase flow patterns across triangular tube bundles for air-water upward flow. Int. J. Multiphas. Flow 80, 43–56. Keeton, L.W., Sinhal, A.K., et al., 1982. ATHOS3: A Computer Program for ThermalHydraulic Analysis of Steam generators. Vol.1: Mathematical and Physical Models and Method of Solution. General Studies of Nuclear Reactors. Lalonde. V., Ross, A., Pettigrew, M.J., Nowlan. I., 2010. Experimental study of dynamic interaction between a steam generator tube and an anti-vibration bar ASME Fluids Engineering Division Summer Meeting, Montreal, Canada, August 01–05, 3, pp. 661–670. Lee, K., Cho, H., Rhee, D.S., et al., 2018. Experimental investigation on the velocity structure and its effect on the flow patterns of air-water two phase flow in a horizontal pipe KSCE. J. Civ. Eng 22, 3363–3372. Li, Shaodan, Tan, Sichao, Chao, Xu, Gao, Puzhen, Sun, Licheng, 2013. An experimental study of bubble sliding characteristics in narrow channel. Int J. Heat Mass Transfer 57, 89–99. Liyan, Tong, Su, Kai, Guo, et al., 2019. Simulation of two phase flow on the secondary side of PWR steam generator. J. Tianjin Univ. (Sci. Technol.) 52, 746–747. Park, I.K., Lee, S.J., Kim, S.H., 2014. Porous media approach of a CFD code to analyze a PWR steam generator. In: The 22th International Conference of Nuclear Engineering, Prague, Czech Republic, July 7–11. Patankar, S.V., Spalding, D.B., 1974. A Calculation Procedure for Transient and Steady State Behavior of Shell-and-Tube Heat Exchangers, Heat Exchanger: Design and Theory Source Book. McGraw-Hill Book Company, New York, USA, pp. 155–176. Quan, Z.T., Zhang, K.T., Xiong, Z.Q., et al., 2019. Experimental study on cross flow induced vibration response of four-span straight tube bundle with TSP or AVB supports. Nucl Eng. Des. 349, 8–19. Shen, Xiuzhong, Schlegel, Joshua P., Hibiki, Takashi, Nakamura, Hideo, 2018. Some characteristics of gas–liquid two phase flow in vertical large-diameter channels. Nucl Eng. Des. 333, 87–98. Suwen, Cui, Yong, Zhu, Hongbin, Ren, 2016. Effect of anti-vibration bar on steam generator tube integrity. Nucl. Power Eng. 37, 109–112. Takai, M., Iwase, T., Uwagawa, S., Nakamura, T. et al., 2000. Flow-induced vibration test of U-bend tube bundle subjected to Freon two phase flow. Part 1: test equipment and partial measured results The 8th International Conference of Nuclear Engineering, Baltimore, MD, USA. Takeshima, K., Fujii, T., Takenaka, N., et al., 2002. The flow characteristics of an upward gas-liquid two phase flow in a vertical tube with a wire coil: Part 1 Experimental results of flow pattern, void fraction, and pressure drop. Heat Transfer-Asian Res. 31, 639–651. Taylor, C.E., Pettigrew, M.J., 2001. Effect of flow regime and void fraction on tube bundle vibration. J. Pressure Vessel Technol. 123, 407–413. Ulbrich, R., Mewes, D., 1994. Vertical, upward water-steam two phase flow across a tube bundle. Int. J. Multiphase Flow 20, 249–272. Vinod, V., et al., 2014. Experimental evaluation of the heat transfer performance of sodium heated once through steam generator. Nucl. Eng. Des. 273, 412–420. Xi-nian, Zang, Shi-fei, Shen, 2003. Nuclear Power Plant System and Equipment. Tsinghua University Press, Beijing, China, pp. 11–30. Yun, B.J., Bae, B.U., Euh, D.J., Park, G.C., Song, C.H., 2010. Characteristics of the local bubble parameters of a subcooled boiling flow in an annulus. Nucl. Eng. Des. 240, 2295–2303.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Major Science and Technology Projects of China (No. 2012ZX06004-012). The authors would like to express their sincere appreciation for the support. References Atkins, M.D., 2016. Chapter 5 – Velocity Field Measurement Using Particle Image Velocimetry (PIV) Application of Thermo-Fluidic Measurement Techniques. Butterworth-Heinemann, pp. 125–166. Bouyahiaoui, H., Azzi, A., Zeghloul, A., Hasan, A., Berrouk, A.S., 2018. Experimental investigation of a vertically downward two phase air-water slug flow. J. Petrol. Sci. Eng. 162, 12–21. Cargnelutti, M.F., Belfroid, S.C., Schiferli, W., Osch, M. Multiphase Fluid Structure Interaction in Bends and T-Joints ASME 2010 Pressure Vessels and Piping Division/KPVP Conference, Bellevue, Washington, USA, July 18–22, 4(2010), pp.75–82. Chu, In-Cheol, Chung, Heung June, Lee, Seungtae, 2011. Flow-induced vibration of nuclear steam generator U-tubes in two phase flow. Nucl. Eng. Des. 241, 1508–1515. Cong, T., et al., 2013. Study on secondary side flow of steam generator with coupled heat transfer from primary to secondary side. Appl. Them. Eng. 61, 519–530. Cong, T., et al., 2014. Three-dimensional study on steady thermohydraulics characteristics in secondary side of steam generator. Prog Nucl. Energy 70, 188–198. Delgado, M., Lee, S., Hassan, Y.A., Anand, N.K., 2018. Flow visualization study at the interface of alternating pitch tube bundles in a model helical coil steam generator using particle image velocimetry. Int. J. Heat Mass Transfer 122, 614–628. Goel, Parul, Nayak, Arun K., Ghosh, Pradyumna, Joshi, Jyeshtharaj B., 2018. Experimental study of bubble departure characteristics in forced convective subcooled nucleate boiling. Exp. Heat. Transfer 31, 194–218. Guangdong Nuclear Power Training Center, 2007. Guangdong Nuclear Power Training Center Devices and Systems of 900MW PWR. Atomic Energy Press, Beijing, China, pp. 87–97. Hetsroni, G., 1982. Handbook of Multiphase Systems. McGraw-Hill Book Company, New York, USA. Hsu, Juei-Tsuen, Ishii, Mamoru, Hibiki, Takashi, 1998. Experimental study on two phase natural circulation and flow termination in a loop. Nucl Eng. Des. 186, 395–409. Ishii, M., Chawla, T.C., 1979. Local drag laws in dispersed two phase flow Nasa Sti/recon Technical Report N, 80. Ishii, M., Hibiki, T., 2010. Thermo-Fluid Dynamics of Two phase Flow. Springer, New York, USA. Ishii, M., Kataoka, I., 1984. Scaling laws for thermal-hydraulic system under single phase and two-phase natural circulation. Nucl. Eng. Des. 81, 411–425. Jeong, J.J., et al., 1999. Development of a multi-dimensional thermal-hydraulic system
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Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Experimental study on the circulating-cavity flow and an innovative central baffle design in a steam generator
T
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Wang Yua,b, Lu Dao-ganga,b, , Wang Congc, Cao Qionga,b, Li Xiang-bina,b, Zhou Shi-lianga,b a
North China Electric Power University, Beijing 102206, China Beijing Key Laboratory of Passive Nuclear Energy Safety and Technology, Beijing 102206, China c Research Institute of Nuclear Power Operation, Hubei, Wuhan 430223, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Circulating-cavity flow Steam-water two phase flow Steam generator Visualization experiment
Steam generator (SG) is the key equipment in pressurized water reactors (PWR), which transfers heat from primary circuit to secondary circuit and has the feed water vaporized into the steam. It is related to the safe, reliable and economical operation of the nuclear power plant. Many researches have been done on SG, including numerical simulation and experimental research. Since it is involved with complicated steam-water (two phase) flow in high temperature and high pressure, it is not easy to measure the key parameters such as pressure, temperature and void fraction, especially to carry out the visual observation. So the detailed working information of SG such as void fraction and flow pattern is still unknown, while this information is very important for the improvement of SG performance. In order to obtain the working information of SG and study the dynamic flow process in secondary side, a visualization scaled-down mock-up experimental bench was set up. Although its operation parameters (temperature and pressure) are much lower than the actual one in PWR, the internal dynamic flow process of SG in this facility is kept similar to the actual one by the scaling analysis and design. Appling the high speed camera, particle image velocimetry (PIV) and self-made optical fiber probes, two phase flow behavior including flow pattern, velocity and void fraction were collected and analyzed in the experiment. From the experiment, a circulation phenomenon is discovered; which is defined as circulating-cavity flow (CCF). At U-bend area, CCF flows from hot side to cold side, which may result in flow-induced vibration (FIV) and jeopardize the U-tubes. In order to limit CCF, a baffle installed in the middle of U-bend area is proposed to suppress CCF. The experimental results show that this baffle can effectively suppress the CCF. This paper may contribute to the design and safety of SG.
1. Introduction In most pressurized water reactors (PWR), the steam generator (SG) is installed vertically, has inverted U-tubes and can do natural circulation (Guangdong Nuclear Power Training Center, 2007), such as the SG in Guangdong Daya Bay Nuclear Power Station (GNPS) shown in Fig. 1 (Liu et al., 2019). SG is the key equipment for heat exchange at primary and secondary coolant circuits in nuclear power plants (NPP). As depicted in Fig. 1, in primary circuits, the coolant with high temperature enters from coolant inlet to inverted U-tubes, and then flows out from coolant outlet. In secondary circuits, the feed water with relatively low temperature enters from feed water inlet to feed water ring and flows through the downcomer (which is between the tube bundle sleeve and the lower shell) to tube sheet, and then feed water flows along the U-tubes. The feed water is heated by the surface of U-tubes,
⁎
which makes most of feed water vaporized to steam. At last, the steam is separated by steam separator and steam dryer from steam-water mixture. In the process of heat transfer, there is complex phase transition at secondary side. In most researches on SG, people most concerned about the heat transfer process between primary circuit and secondary circuit, and the phenomenon caused by steam-water two phase flowing such as flow induced vibration (FIV). As we all know, thermal stress and vibration wear are main reasons causing the damage of SG U-tubes, according to the operation data statistics in NPP (Zang and Shen, 2003), a quarter of unplanned trip was caused by the breakage of SG U-tubes. Thus, in order to acquire the distribution and flow behavior of steam-water two phase flow in secondary side of SG, people mainly used numerical simulation and experimental research to analyze the thermo-hydraulic characteristics of SG. In numerical simulation, most simulations are based on porous
Corresponding author. E-mail address: [email protected] (D.-g. Lu).
https://doi.org/10.1016/j.nucengdes.2019.110495 Received 4 September 2019; Received in revised form 11 December 2019; Accepted 16 December 2019 0029-5493/ © 2019 Elsevier B.V. All rights reserved.
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Generation 2+. The research pointed that the gap between AVB and Utubes, AVB twist and unreasonable insertion depth were critical to wear resistance and would aggravate the FIV. Thus it can be seen that the effective tube support is essential for guaranteeing the service lifetime of U-tubes. There are many people researching the local fluid behavior in the process of heat transfer. Parul et al. (2018) did visualization experiments with high-speed photograph technique to characterize the forced convective nucleate boiling flows. Yun et al. (2010) focused on local two-phase flow parameters through a series of subcooling boiling flow tests in steam-water. Li et al. (2013) studied bubbles sliding behaviors at a visual experiment. There are other excellent researches such as references (Bouyahiaoui et al., 2018; Shen et al., 2018; Lee et al., 2018; Kanizawa and Ribatski, 2016; Takeshima et al., 2002), they most focus on the local region and the flow characteristics inside the tubes, these researches can help us know about the flow patterns on different velocities, the change of void fraction under different mass qualities and so on. But the comprehensive flow behavior in SG especially flow pattern at U-bend area cannot be learned. Furthermore, in actual operation of SG, the main working medium at secondary side is steam-water two phase flow. The research developed in air-water and Freon has quite difference from actual phenomenon. In experiment, direct observation of experimental phenomenon is beneficial to research, but limited by the high temperature and high pressure working condition in SG, the visual research is hard to carry out, so the flow behavior of steam-water two phase flow in SG is still unknown and there is not accurate and comprehensive description. There are few visualization experimental researches on the whole of the SG. Delgado et al. (2018) did a visualization research on a model of helical coil SG using particle image velocimetry (PIV) (Atkins, 2016). But this type of SG is proposed in small modular reactor (SMR), not applied in PWR. In view of the previous researches, it is better to use steam-water working medium and build a visualization experimental bench to observe the detailed flow behavior. Thus, in this study, referring to the prototype SG of GNPS, a scaled-down mock-up experimental bench was constructed to study the dynamic flow process of SG. In order to achieve visualization research, eight glass windows were installed on the bench. And electrical heating rods were utilized to replace primary circuits in SG, which can make water vaporized to steam. The high speed camera, PIV and self-made optical fiber probes were applied to observe and collect data (including flow pattern, velocity and void fraction). The experiment also can provide a benchmark for numerical simulation.
Fig. 1. Structure diagram of GNPS.
media approach which was first proposed by Patankar and Spalding (1974). A lot of simulation procedures such as ATHOS3, MARS, CUPID, and CUPID-SG have been developed and applied (Keeton et al., 1982; Jeong et al., 1999; Jeong et al., 2010; Park et al., 2014). And there are some studies (Cong et al., 2013, 2014) depending on commercial computational fluid dynamics (CFD) code such as ANSYS FLUENT. Although the numerical simulation was excellent, it is not all codes were validated. And in the research of numerical simulation, it is inevitable to use some assumptions and empirical correlations derived from experimental data, which causes that the study results are unreliable. As Ishii and Hibiki (2010) pointed, the unreliable physical models and empirical correlations (for inter-phase friction, heat transfers, and so on) restricted the applicability of the code. For example, in two-fluid model, the physical models and empirical correlations largely depend on the flow regimes. The different assumptions of flow regimes can influence the results. As the references (Hetsroni, 1982; Ishii and Chawla, 1979) showed that the different empirical correlations were for various flow regimes assumptions. Therefore, for widely application, further research on reliable physical models and empirical correlations are needed. Besides, the internal structure of SG is very complex, modeling and meshing are relatively difficult, which also causes it hard to practical application. In experimental research on two phase flow, the tests were usually carried out in nitrogen-water, air-water, Freon and steam-water (Taylor and Pettigrew, 2001; Cargnelutti et al., 2010; Ulbrich and Mewes, 1994; Hsu et al., 1998; Vinod et al., 2014). For example, Chu et al. (2011) conducted the experiment in air-water two phase flow at room temperature and atmospheric pressure condition. Takai et al. (2000) performed the experiment using Freon-123. They both studied the FIV characteristics of U-tubes in SG. There are some other researches focusing on the flow characteristics when tube support was installed. For example, Quan et al. (2019) did experimental study on tube dynamic characteristics and FIV response of tubes with tube support plates (TSP) or anti-vibration bars (AVB). Lalonde et al. (2010) researched the vibration behavior of U-tubes with AVB through experiment, one of the results showed that the vibration of clearance between U-tubes and AVB may lead to ineffective support. And Cui et al. (2016) made analysis about the influence of AVB on integrity of U-tubes based on SG of
2. Experiment 2.1. Experimental bench and test section (1) Experimental bench The experimental bench consists of three major parts: I. Test section (SG model), II. Water supply system, III. Data acquisition system. The water supply system is made up of deionized water tank, feed pump, valve, flowmeter and preheater (shown in Fig. 2). After deionized, the water is saved into water tank. The feed pump and control valve are used to adjust the rate of flow. (2) Test section The test section is scaled down from prototype SG of GNPS based on the scaling analysis which refers to traditional Hierarchical Two-Tiered Scaling (H2TS) method presented by Ishii and Kataoka (1984). During the scaling analysis for primary side, in order to ensure that the model and prototype have the same heat transfer mechanism, the size and structure of U-tubes and tube support plates are same to those of prototype. The friction numbers can be the same by adjusting the damping coefficient. And in secondary side, the time ratio is 1. There are some scaling criteria numbers can meet requirements, but another scaling 2
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Fig. 2. Schematic diagram of experimental bench. Table 1 The main design parameters of SG in GNPS and experimental bench.
Fig. 3. Structure diagram and entity graph of test section. 1-tube sheet; 2, 3, 4, 5, 6, 7-U-tubes containing six electrical heating rods; 8-two groups of anti-vibration bars; 9, 10, 11, 12-four tube support plates.
Parameters
Values of prototype SG
Values of model SG
Thermal load per set Total heat transfer area Pressure of outlet steam The flow of steam Temperature of outlet steam Temperature of feed water The number of U-tubes The material of U-tubes The external diameter of U-tubes The wall thickness of U-tubes The tube pitch The type of tubes support plate
969 MW·t 5429 m2 6.89 MPa 1938 t/h 283.6 °C 226 °C 4474 Inconel 690 alloy 19.05 mm 1.09 mm 27.43 mm Quatre-foil plum blossom
380 kW·t 15.552 m2 0.101325 MPa 529.2 kg/h 100 °C 42 °C 27 304 stainless steel
shape
internal dynamic flow process of SG. We adjusted the experimental parameters and the thermal power was set to 380 kW. This thermal power is too low, so the CHF and Post-CHF phenomena don’t occur. And we just mainly focus on and discover the circulating path; this thermal power doesn’t limit our research. The test section is an about 5.3 m × 2.0 m × 0.1 m sliced box structure depicted in Fig. 3 and the structure of tube support plate is displayed in Fig. 4. The material of Utubes is 304 stainless steel of which the heat-conducting property is
criteria numbers such as phase change number, heat source number and subcooling number that are hard to meet requirements. In order to well revealing the experimental phenomenon, the thermal power should be 6200 kW, which is difficult to achieve in our laboratory conditions. Hence, considering that this experiment places emphasis on the mechanistic study, and the purpose of the experiment is to reveal the
Fig. 4. Structure diagram of tube support plate. 3
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Fig. 5. The PIV system and measuring positions in Velocity measurements.
Fig. 6. The shooting photo by PIV and PIV Image data processing.
4
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Fig. 7. Image of calibrating length. Fig. 9. Schematic diagram of bubble image processing.
similar to that of Inconel 690 alloy. The feed water is deionized water, so the water chemical corrosion is not considered. The detailed experimental parameters are listed in Table 1. The working pressure of secondary side is one bar atmosphere, thus, the saturation temperature of steam is 100 °C, and the feed water is heated to 42 °C by preheater, which ensures the same subcooling temperature of secondary side. The electrical heating rods are installed inside U-tubes and utilized to replace the primary side of SG. In actual operation, from coolant inlet to coolant outlet, the capacity of heat transfer decreases along the U-tubes. Thus, the heating rods were divided into six sections with linearly decreasing heat flux. For observing the flow field, eight glass windows were installed before and after symmetrically. In the experimental bench, all water pipes wrapped with insulation layer but glass windows and stainless steel shell of model SG, so the main heat loss of bench are from the above two parts losing heat. Considering the heat conduction of glasses and shell, heat radiation and convective heat transfer between water and glasses, the total heat loss is calculated to about 5 kW, which was added to test working condition.
(2) PIV data processing In order to get reliable velocity field, each measuring position is shot three times, and the best photograph is selected as a basis for data processing. Fig. 6(a) displays a instantaneous flow field captured by high speed camera in the preliminary test. The tracer particles can be clearly seen. All photos are processed by software Insight 3G and Tecplot to obtain the final analyzable image and data. Fig. 6(b) shows flow field (area inside the red box in Fig. 6(a)) between two U-tubes after data processing. Considering some undesirable data points caused by photo partial distortion, a section of 30 mm × 10 mm between tubes is selected for data processing. The original velocity data from PIV system is just transverse velocity and axial velocity, so resultant velocity has to be 2 2 + vaxi . calculated by formula of vsum = vtrans (3) High speed camera image data processing The experiment applies Fastec TS3 type, 1280 × 1024 pixels high speed camera which can shoot 500 photos in a second. Before experiment, it needs to calibrate length with a ruler (shown in Fig. 7). After experiment, it needs to observe and analyze the motion path of flow field. As Fig. 8 displays, a random bubble moves in a short time △t*(△t*=t2-t1). The two photos at t1 and t2 are processed by using image processing software Image J to measure bubble motion distance, including transverse and axial distance. The time △t* and distances have been get, since then the motion angle, motion displacement and bubble motion velocity can be obtained, thus, from Fig. 9, the general flow direction can be known.
2.2. Measurements and data processing methods 2.2.1. Velocity field measurement (1) PIV system In experiment, PIV system is used to measure the velocity of flow field especially observing bubbles flow behavior with high speed camera. The PIV system mainly consists of measuring devices and tracer fluorescent particles. As shown in Fig. 5(a) and Fig. 5(b), the laser transmitter and high speed camera are put at the same side of the windows and the angle between them is 90°. The angle between incident laser and window is 30°, so is the angle between shooting surface and flow field. When calculating the velocity, the measured transverse velocity should multiply cosine 30°(u*=ucos30°). Some strong reflective areas are covered by a black curtain to reduce the influence of environmental light. The velocity measuring positions (namely 1–10 points marked in Fig. 5(c)) are selected below U-bend area as where the boiling phenomenon becomes obvious; and the high speed camera mainly captures the a-d areas as where the two phase flow phenomenon is most intense.
2.2.2. Void fraction measurement (1) Optical fiber probe Fig. 10 shows the void fraction measuring points in test section. These 12 points are in the third view window (counting from bottom up). The optical fiber probes insert the glass and are fixed to measuring points, which consist of five parts (exhibited in Fig. 11): S1-sensitive head of probe; S2-support tube of optical fiber and sensitive head; S3sealing gland; S4-signal transmission wire; S5-optical connector which
Fig. 8. Schematic diagram of bubble motion. 5
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Fig. 10. Void fraction measuring points.
Fig. 11. Schematic diagram of L-type optical fiber probe.
Fig. 12. Schematic diagram of calibration bench. 1-water pump, 2,5-regulating valve, 3-liquid flowmeter, 4-gas pump, 6-gas flowmeter, 7-pressure relief valve, 8,12-drainage valve, 9-acrylic plates, 10-stainless steel capillary array, 11three-layer stainless steel filter, 13–15-quick-closing valve, 16-drain pipe, 17water tank, 18,19-differental pressure meter.
Fig. 13. Schematic diagram of optical fiber probe.
bypass pipe is opened. The height of water column and air column between two quick-closing valves are measured to calculate the average void fraction. Each probe was calibrated three times. Finally, the average calibration error of all twelve probes is 5.707%, and this error is within the acceptable range, so experiments could be conducted.
connects optical fibers to photoelectron component. In order to make sensitive head of probe fully contact with bubbles in secondary side, part S2 is made into L-type. The part S1 is towards to bottom as to enhance the contact with bubbles.
(b) Void fraction data processing
(2) Optical fiber probe measuring data processing (a) Calibration of optical fiber probe
As Fig. 13 shows, the light is transmitted to the sensitive head of probe. Because refractive indices of gas and liquid are very different, the intensity of returned light varies greatly, which make it easy to distinguish gas and liquid. In optical fiber electric signal processing, when output signal is larger than the threshold value and smaller than the minimum noise voltage, it indicates that this signal is caused by bubbles.
Before using self-made optical fiber probe, it is necessary to calibrate the probe performance. Fig. 12 is the schematic diagram of calibration bench. In measuring area of the calibration bench, the optical fiber probe is installed between the two quick-closing valves. Water and air are utilized to replace steam-water two phase fluid. Keeping water and gas flow rate unchanged and flow pattern in a stable condition, the optical fiber probe is used to measure the average void fraction in a period of time. At the same time, quick-closing valves are closed and the 6
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Table 2 Experimental working condition parameters. Time interval Δt (μs)
Exposure time (μs)
Total flow rate (kg/s)
Initial temperature (°C)
Environmental pressure (Pa)
100 Working condition 1 2 3 4
450 Power (kW) 155 155 255 255
0.147 Flow rate at cold side (kg/s) 0.0735 0.0294 0.0735 0.0294
42 Flow rate at hot side(kg/s) 0.0735 0.1176 0.0735 0.1176
1.01325 × 105 Feed water ratio 1:1 1:4 1:1 1:4
2.3. Experimental test procedure
3.2. Flow phenomenon shot by high speed camera
Prior to the experiment, the tracer particles are injected into the deionized water tank at the ratio of 1:50 and are mixed up. Referring to Fig. 2, the valve and feed pump are firstly opened, according to the reading number of flowmeter, the valve is adjusted to a suitable angle to make the flux 0.147 kg/s. Then preheater and the drain valve of SG are opened, until the temperature of feed water reaches 42 °C, the drain valve is closed. Next the three-way valve is opened, according to reading number of rotor flowmeter, feed water is distributed into cold and hot sides. At the time of water level reaching the required value, feed pump is closed and electric heating rod bundles are started. When boiling phenomenon appearing, the feed pump is opened again; after the flow becomes steady, it is time to collect the flow phenomenon by PIV system and measure void fraction by optical fiber probe. The test was performed at the conditions listed in Table 2. The heat loss of 5 kW was added into all working condition. When collecting void fraction data, the steady state duration is 120 s.
In four shooting areas a-d, every flow field has its general flow tendency. Considering their flow tendency, when operation power is 155 kW and the feed water ratio is 1:1, three groups of bubbles were selected randomly in different times and were recorded their initial and final positions in the same amount of time. All photos are processed according to image processing method mentioned in part 2.2.1, one of these bubbles motion track is shown in Fig. 16. The selected area is wide so that the deflection angles and velocities can reflect the flow information. The bubbles in every group are 5, 10 and 15; due to the big shooting area a, here the selected bubbles are10, 15 and 20. The results show that the parameters in three groups have little difference and the variations are in normal range (shown at Table 3). The deflection angles and transverse velocities of three groups both can reflect flow movement and the following analysis is mainly referring to the second groups. Fig. 16(d) shows that bubble1 is at straight tubes area and is going to flow into U-bend area. At this moment, the fluid has flowed a long distance and has been heated for a while. Small bubbles come together and form larger slug flow. As there is fluid density difference between two sides in transverse direction and bubble buoyancy drags fluid to flow upward, bubble1 not flows along tubes vertically but towards top-right. Average deflection angle of ten bubbles in area d is 60.13°(shown in Table 3), which reflects that two phase flow in d area flows towards top-right. Then, Fig. 16(b) and (c) displays that fluid flows into U-bend area and scours U-tubes transversely, which makes heat transfer suddenly intensify and the number of bubbles increase. The bubbles motion become more actively, and then they join together to form stir flow. It is hard to tell apart a single bubble by this time. Meanwhile, when fluid flows upward, it receives resistance from the Utubes. This resistance is greater than the frictional resistance produced by flowing along the bend tubes. Besides, in the area below U-bend, the driving force resulted by density difference makes two phase fluid flow continuously from cold side to hot side. This driving force and the resistance from U-tubes bring about fluid flow along the bend tubes. When fluid flows into centre position of U-bend, fluid is cut by tubes. Stir flow is cut broken into blocklike flow that can be seen in Fig. 16(a). The transverse velocity at U-bend centre reaches maximum. At the root, middle and centre of U-bend area, average deflection angles of bubbles in the second group are respectively 36.92°, 13.55° and 13.08°, which complementally illustrates that fluid flows along the tubes. On the whole, the two phase fluid generally flows along the bend tubes after going into the U-bend area, and the flow direction is from hot side to cold side.
3. Experimental results and discussion 3.1. Flow characteristic at secondary side of SG As we all know, steam-water two phase flow is the primary phenomenon at secondary side in SG. The velocities of steam-water flow at ten measuring points are displayed in Fig. 14, it can be found that fluid does not completely flow upward along straight tubes, but accompanied with transverse flow. And the velocities of points 1–5 is greater, which is because there is more intense heat transfer near U-bend area that directly affects flowing actively. The velocity streamline shown in Fig. 14 illustrates that fluid generally flows from cold side to hot side, and the transverse velocity increases along the direction of cold side to hot side as shown in Fig. 15. The main reason is that intense heat transfer at hot side produces many bubbles, which results in the density of hot side smaller. So in the large space of the secondary side, density difference brings about transverse flow. At cold side, what is worth noting that the axial velocity of the second row is greater, especially when the feed water ratio (the ratio of feed water injected into the cold side to that into hot side) is 1:1. This is mainly because when two phase fluid flows into U-bend area, fluid scours tubes transversely not just flows upward, which leads to more heat transfer in this area and then steam volume increases quickly. At the moment, the flow resistance of two phase fluid decreases, and massive bubbles carry water to move along U-bend to cold side. This motion generates a downward disturbance which has a big influence on the axial velocity of the first row, so the axial velocity at cold side is smaller. This feature is more obvious at the power of 255 kW. Axial velocities of point 1 and point 2 become negative values, which mean that fluid flows downward. When the feed water ratio is 1:4, heat transfer per unit fluid decreases at hot side but increases at cold side, so the density difference between two sides decreases, which results in transverse velocity tends to stable and the downward disturbance weakens relatively so that the axial velocities of point 1 and point 2 are positive values. As a whole, below the U-bend area, two phase fluid flows from cold side to hot side. And at cold side, there is a downward disturbance from U-bend area.
3.3. Void fraction field distribution The void fraction data of three rows are exhibited in Fig. 17. At the power of 155 kW, the void fraction is 0 at cold side, which may be because there is not obvious boiling at cold side so that departure from nucleate boiling (DNB) points only appears on several heating tube walls, and these generated bubbles are not fully developed, thus, in steady measuring stage, the measured data is 0. In comparison, heat transfer becomes more intense at the power of 255 kW; as a consequence, void fraction at cold side has a bit of promotion. And when 7
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Fig. 14. Velocity of ten measuring points. 8
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Fig. 15. Component velocity in transverse and axial directions.
the feed water ratio is 1:4, at cold side, void fractions of row1 and row2 are more than 10% and void fraction of row3 is at around 5%. This is as feed water is less at cold side; the heat transfer per unit fluid increases, which leads to more intense boiling. At hot side, heat transfer is relative much; numerous bubbles generate and combine together, so steam volume gradually increases. Due to the fact that heat transfer per unit fluid at hot side is more under the feed water ratio of 1:1, in this condition, void fraction changes faster. Table 4 displays average void fraction data. At the same power, void fraction distribution unevenness decreases with increase of uneven feed water ratio, that is to say, more feed water are injected into hot sides that can make the transverse void fraction field become uniform. As a whole, under the influence of fluid density difference in secondary side and more heat transfer at hot side, the void fraction at hot side is more than that at cold side.
in this large space brings about orderly flowing. When two phase fluid flows into U-bend area, resistance from bend tubes and driving force from density difference compels fluid to flow along bend tubes, here plenty of steam goes into steam separator, other two phase fluid flows toward cold side. According to the fluid motion parameters of shooting areas in Fig. 18(a), the fluid from the cold side would flows downward, and meanwhile they meet the below fluid which brings upward momentum, this momentum impedes the flow and compels them to change flow direction, so the fluids from two directions join together and then flow to hot side continually in the process of heat transfer. In the flow process, there is an obvious cyclic flow phenomenon at the U-bend area (shown in Fig. 18(b)), which is called circulating-cavity flow (CCF). The CCF may result in FIV and do harm to heat transfer tubes.
3.4. Analysis of circulating-cavity flow phenomenon
In order to suppress the CCF at U-bend area discovered in experiment, two types of central baffle are designed and set in the middle of U-bend. The central baffles are 5 mm thick steel plates, detailed parameters and installation positions are shown in Fig. 19. Through above analysis, when the feed water injected into two sides is the same, two phase fluid flows more intensely, hence FIV is more severe. Thus, based on this feed water ratio condition, the suppressing CCF test was performed. The flow phenomenon before and after installing central baffles are displayed in Fig. 20 and Fig. 22, where the red circle marks the position of one bubble at t1 and t2; the transverse velocity contrast results are shown in Figs. 21 and 23. As Fig. 20 displays, at the power of 155 kW, after installing baffle, most fluid is blocked to the hot side. Because the gaps between tubes are not sealed in baffle 1, there are some bubbles flowing to cold side through these gaps. In comparison, Fig. 20(c) shows that the bubbles
4. Suppression of the circulating-cavity flow
Through the above analysis, two phase fluid flow tendency in secondary side is summarized and estimated as Fig. 18 depicts. The feed water experiences a complex flow process in the secondary side of SG. First, feed water flows into downcomer of test section and then comes into contact with U-tubes. At this moment, fluid is still water. Then, fluid flows along the U-tubes and is heated a period of time. The degree of subcooling gradually decreases, when it reaches 0, namely the temperature of fluid reaches saturation temperature; a few bubbles appear on the wall of some individual tubes. Along with heat transfer increasing, more bubbles appear and combine together. As flow resistance of gas is small, bubbles could drag water flow upward. Meanwhile, at hot side, more bubbles and high temperature make two phase fluid density becomes smaller than that at cold side. Density difference 9
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Fig. 16. Bubble motions at four shooting area of U-bend area. Table 3 Average velocity of three groups of bubbles. Position/Bubble numbers
Displacement (cm)
Velocity (cm/s)
Transverse velocity (cm/s)
Deflection angle (°)
Time internal (ms)
a (10/15/20) b (5/10/15) c (5/10/15) d (5/10/15)
6.59/7.06/6.72 2.96/2.97/2.64 3.54/3.27/3.67 3.45/3.11/3.22
82.40/88.74/83.97 49.25/49.41/43.92 58.96/54.41/61.25 37.60/31.06/34.19
69.41/76.26/70.64 48.09/47.59/41.73 31.99/32.62/32.64 13.16/15.17/15.28
14.34/13.08/14.59 9.98/13.55/13.37 37.54/36.92/38.29 69.54/60.13/60.28
80 60 60 100
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Fig. 17. Void fraction of three rows.
transverse velocity decreases from 110.84 cm/s to 75.59 cm/s and 30.56 cm/s. In this test section, there are two groups of AVBs, and the baffle 1 is similar to AVB in structure. They both have gaps in radial direction after installed among the U-tubes. On the contrary, the baffle 2 is designed that it can fill up these gaps. Through the above velocity data and bubbles flow phenomenon, it can be found that the AVBs can weaken the CCF, but the suppression effect is not very good because there is fluid flowing through gaps. However, the baffle 2 obviously can suppress the transverse flowing well. Expect for supporting the U-tubes, this kind of baffle can both weaken vibration and reduce the flow channels. The baffle 2 can be considered as an optimization design in the future designs work.
Table 4 Average void fraction data. Power (kW)
155 255
Feed water ratio
1:1 1:4 1:1 1:4
Transverse average void fraction (%) Row 1
Row 2
Row3
Overall average value
10.972 9.698 23.813 15.713
9.605 7.139 19.231 10.880
4.415 3.819 12.449 7.004
8.331 6.885 18.497 11.199
flowing to cold side is less in SG with baffle 2. In Fig. 21, the transverse velocity decreases from 70.92 cm/s to 28.99 cm/s and 19.33 cm/s respectively after installing baffle 1 and baffle 2, which illustrates that the suppressing function of baffle works well and the restraining transverse flow effect of the baffle 2 is more significant. At the power of 255 kW, the two phase fluid flows more intensely, thus, more bubbles flow to cold side driven by the CCF (shown in Fig. 22), although like this, the baffle still has a certain suppressing effect. As Fig. 23 displayed, the
5. Conclusions The main conclusions of this present study are summarized as follows.
Fig. 18. Schematic diagram of the CCF flow behavior. 11
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Fig. 19. Two types of central baffle.
Fig. 20. Image of two phase flow behavior before and after installing central baffles (P = 155 kW).
12
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Fig. 21. Transverse velocity contrast on three conditions (P = 155 kW).
Fig. 23. Transverse velocity contrast on three conditions (P = 255 kW).
(1) A visualization scaled-down mock-up experimental bench is set up, which can reveal the internal dynamic flow process of SG and discover the circulation path of fluid in SG. (2) The CCF is discovered in experiment, which is a circulating flow phenomenon at U-bend area. Its detailed flow behavior (including flow pattern, velocity and void fraction) is obtained by PIV and selfmade optical fiber probes quantificationally. (3) Since the CCF may cause FIV and have bad impact on U-tubes. A baffle plate installed in the middle of U-bend area is newly proposed to suppress the CCF. The experimental results show that the baffle
can effectively suppress the CCF. Thus, the designer can consider designing a suitable baffle in SG to suppress vibration of U-tubes. CRediT authorship contribution statement Yu Wang: Data curation, Formal analysis. Dao-gang Lu: Conceptualization, Supervision, Visualization. Cong Wang: Investigation, Data curation, Formal analysis. Qiong Cao: Project administration. Xiang-bin Li: Formal analysis. Shi-liang Zhou: Resources.
Fig. 22. Image of two phase flow behavior before and after installing central baffles (P = 255 kW). 13
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Declaration of Competing Interest
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